DTE vs DCE
This is one of the most misunderstood areas of RS-232. DTE stands for Data
Terminal Equipment, and DCE stands for Data Circuit-terminating Equipment or
Data Communications Equipment. The DTE is typically either a dumb terminal or
the serial port on a computer/workstation. The DCE is typically a modem, CSU/DSU,
or other piece of data communications equipment.
Where it gets confusing is when you start to talk about signal definitions and direction. For example, it's easy for someone to understand that when you transmit data, you send it out. However, when you talk in terms of the DCE, it becomes an input. This is because the specification was written from the perspective of the DTE end of the link. Another example is that the Receive Data signal is an input to DTE, but an output from DCE. See Table 1 below.
|DTE Signal Direction||Signal||DCE Signal Direction|
Therefore, a straight through "one to one" cable is all that is necessary between a modem and a standard DTE serial port. However, if you want to connect two DTE ports together, you have to simulate the existence of the pair of DCE devices, typically modems, that would normally be between the two DTE devices. This is where the null-modem device or cable comes in. See our cabling guide for more info.
- Signal State Voltage Assignments (see figure 1 below):
- Voltages of -3v to -25v with respect to signal ground (pin 7) are considered logic '1' (the marking condition), whereas voltages of +3v to +25v are considered logic '0' (the spacing condition). The range of voltages between -3v and +3v is considered a transition region for which a signal state is not assigned.
|Pin Number||ITU-T Number||Circuit||Description|
|2||103||BA||Transmitted Data (TxD)|
|3||104||BB||Received Data (RxD)|
|4||105/133||CA/CJ||Request to send/ready for receiving (RTS)|
|5||106||CB||Clear to send (CTS)|
|6||107||CC||DCE ready (DSR)|
|7||102||AB||Signal common (SG)|
|8||109||CF||Received line signal detector (DCD)|